[ODRHash] Merge the two function hashes into one.

Functions that are a sub-Decl of a record were hashed differently than other
functions.  This change keeps the AddFunctionDecl function and the hash of
records now calls this function.  In addition, AddFunctionDecl has an option
to perform a hash as if the body was absent, which is required for some
checks after loading modules.  Additional logic prevents multiple error
message from being printed.

llvm-svn: 336632